Addition product, production and use thereof as corrosion inhibitor

ABSTRACT

The invention relates to an addition product that can be produced from hexafluorosilicic acid, hexafluorotitanic acid, and/or hexafluorozirconic acid by an acid-base reaction with one or several organic bases and a method for production and use thereof. The addition products according to the invention guarantee a rapid and lasting inhibition of corrosion processes; they are in particular suitable for inhibiting the corrosion of light metals.

TECHNICAL FIELD

[0001] The invention relates to an addition product, its production fromone or several organic bases and one or several of the acidshexafluorosilicic acid, hexafluorotitanic acid, and hexafluorozirconicacid, and its use as corrosion inhibitor, in particular for theprotection of light metals.

PRIOR ART

[0002] For the corrosion protection of light metals or light metalalloys, conversion baths with the corrosion inhibitors hexafluorosilicicacid, hexafluorotitanic acid, and hexafluorozirconic acid are used. Thelight metals are treated by immersing them in the acid bath for a giventime. A conversion layer forms hereby that protects the treated metalfrom corrosion. For prior art, reference is made here to WO 9967029 byway of example.

[0003] Such methods have the disadvantage that it is very laborious tocarry out the process. Various cleaning and rinsing baths precede andfollow the actual treatment in the acid bath. However, it is primarilydisadvantageous that the conversion layer contains only a small contentof the ions with inhibiting effect. Because of this, the duration of theprotective effect is limited.

[0004] The protective effect can be lengthened by the metal to beprotected having a coating, e.g., a lacquer or adhesive, that representsa reservoir for the ions with inhibiting effect. If the coating isdamaged, the inhibitor can be dissolved from the coating by arrivingwater and transported to the damaged point, in order to have aninhibiting effect there. As inhibitors, anti-corrosive pigments such aszinc phosphate or barium metaborate are customarily added for thispurpose. Metal salts of hexafluorosilicic acid, hexafluorotitanic acid,and hexafluorozirconic acid can likewise be incorporated into polymersin the form of pigments.

[0005] However, the above-named salts have the disadvantage that theirsolubility in water is not very high and their suitability for a rapidprotection of a damaged point in the metal is therefore low. Inaddition, the efficacy potential of these salts is lower than that ofthe particularly effective chromates.

[0006] Chromates are known as particularly effective inhibitors. Becauseof this, it is possible to use even relatively sparingly solublechromates as inhibitors. Chromates used particularly frequently are zincchromate, barium chromate, and strontium chromate, as well as mixturesof these compounds.

[0007] However, due to the high toxicity of chromium compounds(hexavalent chromium is carcinogenic and mutagenic) it is necessary toreplace them by less toxic substitutes.

STATEMENT OF THE INVENTION

[0008] It is the object of the present invention to overcome thedisadvantages of the prior art and to make available corrosioninhibitors that guarantee a rapid and lasting inhibition of corrosionprocesses.

[0009] The object according to the invention is attained by the additionproduct according to claim 1,. the method for its production accordingto claim 9, its use according to claims 10 through 17, and the componentaccording to claim 18. Dependent claims disclose advantageousdevelopments.

[0010] The addition products according to the invention can be producedfrom hexafluorosilicic acid, hexafluorotitanic acid, and/orhexafluorozirconic acid by an acid-base reaction with one or severalorganic bases.

[0011] Salts with the anions hexafluorosilicate, hexafluorotitanate,and/or hexafluorozirconate and cations from the protonated organic basesare obtained by these means. An organic base in the sense of thisinvention is generally understood to mean organic compounds that whenreacted with inorganic acids such as in particular hexafluorosilicicacid, hexafluorotitanic acid, or hexafluorozirconic acid, yield salts orsalt-type compounds. The following can be given as a general reactionequation:

m*B+n*H₂EF₆→(BH⁺)_(2n)(EF₆ ²⁻)_(n)+(m−2n)*B

[0012] where E=Si and/or Ti and/or Zr;

[0013] B designates the organic base; if the organic base carries morethan one basic group, the basic group is understood to mean B;

[0014] m and n are integers, whereby m≧2n; for non-polymer compounds inparticular, m=2n.

[0015] According to the invention, an organic base is preferablyunderstood to mean organic nitrogen-containing compounds. Furthermore,due to improved handling, it is particularly preferred that these areunderstood to mean non-cyclic bases with a molecular weight greater than105 g*mol⁻¹, and cyclic bases with a molecular weight greater than 70g*mol⁻¹.

[0016] Compared with metal salts of hexafluorosilicic acid,hexafluorotitanic acid, or hexafluorozirconic acid, the additionproducts according to the invention offer the advantage that theyinhibit not only the anodic partial reaction but also, due to theorganic constituent of the addition product according to the invention,the cathodic partial reaction of a corrosion process and thus result inan increased corrosion inhibition. It is mostly contaminants or morenoble light metal alloy constituents that act as the cathode hereby; theanodic partial reaction takes place at the non-noble points of thealloy, mostly phases with a high proportion of the light metals. Thestarting point of the corrosion is as a rule a point at which damage tothe coating of the light metal or the light metal alloy has taken place.

[0017] Compared with metal salts of hexafluorosilicic acid,hexafluorotitanic acid, or hexafluorozirconic acid, the additionproducts according to the invention also offer the advantage that theyare more readily soluble in aqueous media. Because of this, the anionSiF₆ ²⁻, TiF₆ ²⁻, or ZrF₆ ²⁻ needed for the anodic inhibition are madeavailable more rapidly and can be dissolved from the coating morerapidly by arriving water and be transported to the damaged point inorder to have an inhibiting effect there. Moreover, the additionproducts according to the invention have the advantage that they do notrepresent carcinogens such as hexavalent chromate compounds or toxicheavy metal compounds such as barium salts.

[0018] Surprisingly, it was found that in contrast to the free acids,the addition products according to the invention can be incorporatedinto polymers and can therefore be used for the production of protectivelayers with an inhibiting effect. As polymers, the basic constituents oflacquers, adhesives, primers, paints, or embedding compounds are to benamed in particular hereby. The addition to a polymer has the advantagethat a reservoir of the corrosion inhibitor is made available thatprevents the corrosion under the polymer when corrosive media diffuse inor at damage points. Epoxy resins, urethanes, acrylates, alkyd resins,or polyvinyl acetate and its copolymers are named as polymers by way ofexample. However, the inhibitors according to the invention not only canbe incorporated into polymers but like hexafluorosilicic acid,hexafluorotitanic acid, or hexafluorozirconic acid, also can be used inconversion baths and other liquids for the treatment of light metalsurfaces.

[0019] Preferred organic bases for the production of the additionproducts according to the invention are heterocyclic nitrogen compounds.

[0020] Polymer and non-polymer heterocyclic nitrogen compounds formstable complexes with heavy metals. Through the complex formation, thecathodic partial reaction of the corrosion process is blocked. Toproduce the corrosion inhibitors according to the invention,heterocyclic nitrogen compounds are therefore preferred that are goodcomplex-formers with metals. Heterocycles that form stable complexeswith contaminants or nobler alloy constituents such as copper, which isfrequently present in light metal alloys, are particularly advantageous.

[0021] For the production of particularly strongly inhibiting additionproducts, organic bases are used that feature at least two heteroatoms(such as N, O, S) in one or two heterocyclic rings. Preferred organicbases are heterocyclic nitrogen compounds such as, e.g., substituted andunsubstituted 1,2,4-triazoles, 1H-benzotriazoles, benzothiazoles,benzimidazoles, benzoxazoles, 2,2′-biquinolines, nitrones,2,5-dimercapto-1,3,4-thiadiazoles, and 2,9-dimethylphenanthrolines.

[0022] Particularly preferred heterocyclic nitrogen compounds with aparticularly strongly inhibiting effect are, e.g., 1H-benzotriazole,5-methylbenzotriazole, 5-carboxybenzotriazole, benzothiazole, 2-alkylbenzothiazole, 2-mercaptobenzothiazole, 2-mercaptobenzothiazolesuccinicacid, benzimidazole, 2-alkylbenzimidazole,2-(5-aminopentyl)benzimidazole, benzoxazole, and 2-mercaptobenzoxazole.Heterocycles with alkyl side-groups reduce the water solubility and thusimprove the long-term effect of the corrosion inhibitor.

[0023] Particularly preferred organic bases are also compounds such assubstituted and unsubstituted phthalocyanines, chlorines, andporphyrins. Due to the formation of very stable complexes with a largenumber of metals and thus the strong corrosion inhibiting effect, thesecompounds are particularly suitable for the production of the additionproducts according to the invention.

[0024] In a preferred form, the organic base for the production of theaddition products according to the invention is a polymer with one orseveral basic groups. A basic group is understood in particular. herebyto mean the above-mentioned heterocyclic compounds, but quite generallyalso polymers with short side-chains such as, for example,polyvinylamine.

[0025] The addition products according to the invention are alsounderstood to mean those produced from polymer bases whose basic groupsare only partially protonated. Copolymers can also be used as polymers.For example, derivatives of the corresponding monomer organic bases thatfeature a vinyl group can be polymerized together with a comonomer inorder to obtain suitable polymers. By the selection of the comonomer,for example the solubility in water or the compatibility withconstituents of a polymer matrix into which the addition product is tobe incorporated, can be influenced. Thus, for instance, the watersolubility can be reduced by raising the proportion of the alkylside-groups of the polymer and thus the long-term effect can be improvedduring use as a corrosion inhibitor; by using vinylimidazole monomers,the water solubility can be raised and a good short-term effectachieved.

[0026] Polymers are also used advantageously when in addition tocorrosion-inhibiting properties it is also desired that a proton bufferpermanently fixed in the polymer be present. This buffer binds acidformed. during the corrosion. A strong buffer effect is achieved byusing addition products. in which not all basic groups are protonated.

[0027] Surprisingly, it was found that an additional adhesion-promotingeffect can be achieved by polymers and copolymers. Through thisadditional adhesion-promoting effect a better adhesion to a substrate isachieved of a polymer matrix into which the addition product accordingto the invention was incorporated or of the protective layer with aninhibiting effect produced therefrom. In general, polymer-based additionproducts according to the invention offer a lasting protection ofsurfaces against corrosion.

[0028] In a particular embodiment, the addition products according tothe invention are used together with metal salts with the anions SiF₆²⁻, TiF₆ ²⁻, and/or ZrF₆ ²⁻ and/or additionally phosphates, borates,vanadates, molybdates, wolframates, and/or phosphonates.

[0029] With these previously known corrosion-inhibiting metal salts withthe anions SiF₆ ²⁻, TiF₆ ²⁻, and/or ZrF₆ ²⁻ and/or conventionalanti-corrosive pigments based on phosphates, borates, vanadates,molybdates, wolframates, and phosphonates, it is preferred that themetal be selected from the group comprising potassium, sodium, zinc,barium, strontium, calcium, magnesium, manganese, cobalt, nickel, tin,zirconium, aluminum, and the rare earth metals. Barium, strontium, andcalcium, and, depending on the application case, also zinc, areparticularly suitable hereby.

[0030] The use of the addition products according to the invention incombination with previously known corrosion inhibitors or anti-corrosivepigments is preferred in particular when a synergistic effect results inthe specific application case. This is, e.g., the case when the cathodicpartial reaction of the corrosion process is already adequatelyinhibited by a small amount of the addition product according to theinvention, but this amount is not sufficient for the inhibition of theanodic partial reaction. In such a case according to the invention apreviously known inhibitor that is capable of inhibiting the anodicpartial reaction is used in addition.

[0031] Furthermore the combination with previously known corrosioninhibitors offers the advantage that mixtures of various inhibitors canbe produced that feature a broad range of application. By means ofaddition products according to the invention with a high watersolubility, and previously known corrosion inhibitors or additionproducts according to the invention with a moderate solubility in water,mixtures are obtained that feature both a rapid anti-corrosive effect(primary effect) and a long-term effect (secondary effect).

[0032] The addition products according to the invention are suitable inparticular for use as corrosion inhibitors.

[0033] The corrosion-inhibiting effect of the addition productsaccording to the invention is proved, e.g., by dissolving the additionproducts in water together with a corrosive substance and allowing theresulting solution to act on the metals to be protected. In order toachieve an adequate differentiation, the content of the corrosivesubstance is selected to be higher than that of the inhibitor.

[0034] For use as corrosion inhibitors, the addition products accordingto the invention are advantageously incorporated into one or severalpolymers. Such compositions are used to produce protective layers withan inhibiting effect. In particular, polymers are understood hereby tomean lacquers, adhesives, primers, embedding compounds, sealants,paints, and/or anti-corrosive primers. The addition to a polymer has theadvantage that a reservoir of the corrosion inhibitor is made availablethat prevents the corrosion under the polymer when corrosive mediadiffuse in or at damage points. Epoxy resins, urethanes, acrylates,alkyd resins, or polyvinyl acetate and its copolymers are named aspolymers by way of example. The corrosion-inhibiting addition productscan also be used quite generally in protective layers of any kind.

[0035] Furthermore, the addition products according to the invention areused advantageously in conversion baths. However, they can be used quitegenerally in aqueous solutions for the treatment of light metalsurfaces.

[0036] They have the advantage hereby that in addition to the inhibitingof the anodic partial reaction of the corrosion by means of the SiF₆ ²⁻,TiF₆ ²⁻, or ZrF₆ ²⁻, an inhibiting of the cathodic partial reaction cantake place by means of the organic bases of the conversion layer. Whenused in a chemical bath, the inhibitors according to the invention canalso be produced as intermediates by placing the correspondingcomponents in the bath.

[0037] The addition products according to the invention are preferablyused for the corrosion protection of components made of aluminum,magnesium, zinc, and/or alloys of these elements and in particular forthe corrosion protection of components made of copper-containing lightmetal alloys. The following are mentioned as alloys by way of example:the aluminum alloys AA2024, AC120, and the magnesium alloys AM50. Due totheir high strength, the copper-containing aluminum alloys arefrequently used in aircraft construction. The corrosion protection ofcomponents for aircraft construction is therefore a preferred field ofapplication of the inhibitors according to the invention. The corrosioninhibitors according to the invention can be used here both in treatmentliquids for the components and in lacquers, primers, adhesives,sealants, or embedding compounds.

[0038] A further preferred field of application of the corrosioninhibitors according to the invention is the corrosion protection ofcomponents exposed to weathering. These include in particular componentsfor motor vehicle construction.

[0039] Components are also claimed that feature a light metal and/or alight metal alloy as well as corrosion-inhibiting constituents, wherebythe latter comprise one or several bases, preferably bases describedabove in more detail, that inhibit the cathodic partial reaction ofcorrosion processes on this component. Components that have been adheredwith a composition containing a corrosion inhibitor are not claimed.

EXAMPLES OF USE

[0040] Without restriction of the generality, the corrosion inhibitorsaccording to the invention are explained in more detail below on thebasis of examples of use.

Example 1

[0041] Synthesis of 1H-benzotriazole Hexafluorosilicate

[0042] 100.1 g of 1H-benzotriazole is added under stirring over thecourse of half an hour to 242 g of a 25% aqueous solution ofhexafluorosilicic acid. The first portions dissolve and become clear. Inthe later stage of the reaction, the resulting reaction product partlyprecipitates again already. After addition of the 1H-benzotriazole iscomplete, 300 mL of 96% ethanol is added and the mixture is stirred fortwo hours. The resulting precipitate is filtered off in a Büchner funneland the filter cake is dried under vacuum at 50° C. until constantweight is achieved. In order to obtain further product, the filtrate isconcentrated to half and the precipitate is treated in the same. way asthe main amount of the product. The dried product is ground in the ballmill and is screened with a 30 μm screen.

Example 2

[0043] Synthesis of 1H-benzotriazole Hexafluorotitanate

[0044] The synthesis is carried out as described in Example 1, with 115g of a 60% hexafluorotitanic acid being used instead of thehexafluorosilicic acid.

Example 3

[0045] Synthesis of 1H-benzotriazole Hexafluorozirconate

[0046] The synthesis is carried out as described in Example 1, with 193g of a 45% hexafluorozirconic acid being used instead of thehexafluorosilicic acid.

Example 4

[0047] Testing of the Corrosion-Inhibiting Properties

[0048] The corrosion-inhibiting properties are tested in comparison withthe conventional inhibitors sodium chromate, barium metaborate, andyttrium vanadate. For comparison, the corrosive effect of a sodiumchloride solution not provided with an inhibitor is determined.

[0049] The corrosion-inhibiting properties are tested using thecataplasm test. The metal sample is first degreased with methyl ethylketone, polished with 100-grain abrasive paper, and cleaned once againwith methyl ethyl ketone. The sample is wrapped in cotton wool andsaturated with the test solution. This prepared sample is stored in atightly sealed vessel for 1344 hours ( 8 weeks) at 60° C. The testsolution is made of 1% by wt sodium chloride as a corrosive substanceand 0.1% by wt of the respective corrosion inhibitor in demineralizedwater. Aluminum AA2024 (size 20×80×1 mm), aluminum AC120 (size 25×100×1mm), and magnesium AM50 (size 25×100×3 mm) were used as samples.

[0050] In addition to examples of the corrosion inhibitors according tothe invention, commercially available corrosion inhibitors and asolution without corrosion inhibitor were tested as comparativeexamples. The samples are evaluated based on the corroded area and thenumber of pits formed. The results are shown in the following Table:Magnesium Aluminum Aluminum Inhibitor Example AM50 AA2024 AC120 NoneComparative 90% 100%, 90%, >10 pits 1 pit, efflo- rescences Sodiumchromate Comparative 15%  40%, 10% 1 pit* 1 H-benzotriazole According to20%  30%, 10% hexafluorosilicate the invention 1 pit* 1 H-benzotriazoleAccording to 60%  40%, 20% hexafluorotitanate the invention 2 pits* 1H-benzotriazole According to 60%  30%, 20% hexafluorozirconate theinvention 3 pits* Barium metaborate Comparative 50% 100%, 40%, 10 pitsefflo- rescences Yttrium vanadate Comparative 60% 100%, 80%, >10 pitsefflo- rescences

[0051] The results show that the inhibitors according to the inventionare as good as the chromate, and develop a greater efficacy than theother commercially available corrosion inhibitors.

1. Addition product that can be produced from hexafluorosilicic acidand/or hexafluorotitanic acid and/or hexafluorozirconic acid by anacid-base reaction with one or several organic bases.
 2. Additionproduct according to claim 1, characterized in that the organic base isa heterocyclic compound.
 3. Addition product according to claim 2,characterized in that the organic base is selected from the groupcomprising 1,2,4-triazole, 1H-benzotriazole, benzothiazole,benzimidazole, benzoxazole, 2,2′-biquinoline, nitrone,2,5-dimercapto-1,3,4-thiadiazole, and 2,9-dimethylphenanthroline, aswell as derivatives of these parent substances.
 4. Addition productaccording to claim 3, characterized in that the organic base is selectedfrom the group comprising 1H-benzotriazole, 5-methylbenzotriazole,5-carboxybenzotriazole, benzothiazole, 2-alkylbenzothiazole,2-mercaptobenzothiazole, 2-mercaptobenzothiazolesuccinic acid,benzimidazole, 2-alkylbenzimidazole, 2-(5-aminopentyl)benzimidazole,.benzoxazole, and 2-mercaptobenzoxazole.
 5. Addition product according toclaim 2, characterized in that the organic base is selected from thegroup comprising phthalocyanine, chlorine, and porphyrin, as well asderivatives of these parent substances.
 6. Addition product according toclaims 1 through 5, characterized in that the organic base is a polymerwith one or several basic groups.
 7. Composition containing one orseveral addition products according to one or several of claims 1through 6 as well as one or several metal salts with the anions SiF₆ ²⁻,TiF₆ ²⁻, and/or ZrF₆ ²⁻.
 8. Composition containing one or severaladdition products according to one or several of claims 1 through 6 aswell as phosphates, borates, vanadates, molybdates, wolframates, and/orphosphonates.
 9. Method for the production of the addition productaccording to claims 1 through 6, characterized in that hexafluorosilicicacid and/or hexafluorotitanic acid and/or hexafluorozirconic acid aswell as one or several organic bases undergo an acid-base reaction. 10.Use of an addition product according to claims 1 through 6 as acorrosion inhibitor.
 11. Use according to claim 10, characterized inthat the. addition product is incorporated into one or several polymers.12. Use according to claim 10 in conversion baths.
 13. Use according toclaim 11, characterized in that the polymer is a lacquer, an adhesive, aprimer, an embedding compound, a sealant, a paint, and/or ananti-corrosive primer.
 14. Use according to claims 10 through 13 for theprotection of components made of copper-containing light metal alloys.15. Use according to claims 10 through 14 for the protection ofcomponents made of aluminum, magnesium, zinc, and/or alloys of theseelements.
 16. Use according to claim 15 in aircraft construction. 17.Use according to claims 10 through 16 for the protection of componentsthat are exposed to weathering.
 18. Component comprising a light metaland/or a light metal alloy as well as corrosion-inhibiting constituents,characterized in that the corrosion-inhibiting constituents comprise oneor several bases, in particular according to claims 2 through 6, thatinhibit the cathodic partial reaction of corrosion processes on thiscomponent, whereby components that have been adhered with a compositioncontaining a corrosion inhibitor are not claimed.